Fresh water from rivers or lakes is treated in a water plant to convert the raw water into water for human consumption meeting accepted purity standards. The processes used have been honed over the years to produce high quality potable water from raw water at surprisingly low costs.
The primary treatment in a water plant occurs in a clarifier where a flocculant is added to raw water. A water plant clarifier is a large sized, round or rectangular concrete structure. The flocculant causes particles suspended in the water to coagulate, subsequently growing in size and weight. The clarifier is sized to provide sufficient residence time for the majority of the solids to drop out of suspension. Water is then passed through sand filters, perhaps treated with activated carbon, chlorinated and possibly fluoridated before being delivered to water supply mains which transport the treated water to residences, businesses and industries.
One of the recurrent problems in water treatment plant operations is the growth of algae in the clarifier and sand filters. Algae come in many types including filamentous algae, such as Cladaphora and Spirogyra, planktonic algae such as Microcystis and Anabaena, branched algae such as Chara vulgaris and Nitellam, swimming pool algae commonly referred to as black, brown and red algae and algae found in ponds such as Dictyosphaerium, Spirogyra, Oedogonium, Chlorococcum, Pithophora, Hyudrodictyon and Lyngbya. It is not uncommon to see a municipal water plant clarifier with a beard of algae around its peripheral walls and filamentous algae growths several feet long.
As used herein, the term municipal water plant is intended to mean a water plant used in treating raw water and converting it to potable water for human consumption, regardless of whether the entity doing so is public or private.
Algae blooms have been noted to appear literally overnight under the right temperature and sunlight conditions and, if left untreated, will cause taste and odor problems in the finished waters. The problems caused by algae in municipal water plants are handled in a variety of ways by current treatment methods. The odor and taste problems which typically recur during periods of high summer temperatures and long daylight hours occur from detritus thrown off by algae in the clarifier. Not all of this detritus is removed by sand filters. Any detritus passing through the sand filters is converted in the final chlorination process to a family of chloro-organic compounds which cause the objectionable smell and taste that consumers complain about.
The standard treatment for controlling algae in municipal water plants is to scatter crystals of cupric sulfate pentahydrate, CuSO4.5H2O, which is also known by its common name blue vitriol, into the clarifier. Blue vitriol is commercially available in 50 pound bags having crystals ranging in size from fine (xe2x85x9xe2x80x3) to large (1xe2x80x3). Scattering is done with a shovel, a scoop, or by hand. Ideally, the crystals dissolve in the water so the copper ion is present in the water. The soluble or active copper (II) ion kills algae because of its effect on chlorophyll which is a large porphyrin structure occurring either as blue-green chlorophyll-a or yellow-green chlorophyll-b. Both molecules have four centrally placed nitrogen atoms which complex a single magnesium atom. The magnesium removes carbon dioxide from the water and delivers it to the algae thus allowing photosynthetic growth. The soluble copper (II) ion replaces the magnesium by forming a stronger porphyrin complex, which does not bond with carbon dioxide. The algae dies by virtue of its growth mechanism being squelched by a lack of carbon dioxide, in a process analogous to the chemical poisoning of hemoglobin in mammals. A small part of the algicidal copper exits the treated water stream in the clarifier sludge and not with the finished water because it has been intimately bonded to the algae chlorophyll. A large part of the copper sulphate is believed to remain undissolved and drops into the clarifier sludge as copper hydroxide coated pellets. One of the inherent advantages of copper algicides is that algae cannot mutate or evolve to avoid its effect. No amount of evolution can prevent copper from displacing magnesium in the chlorophyll and no amount of evolution can cause the copper porphyrin to absorb carbon dioxide.
Disclosures of some interest are found in U.S. Pat. Nos. 3,844,760; 4,012,221; 4,505,734 and 5,541,150.
The above description of the prior art is an idealized situation but which has a number of practical problems and disadvantages, some subtle and some not so subtle. A substantial part of the blue vitriol does not dissolve because it is difficult to dissolve in water which is not acidic. Plainly put, blue vitriol crystals do not dissolve very well in pH 7, or more alkaline, water. Thus, much of the copper sulfate is wasted because it ends up in the clarifier sludge as blue vitriol pebbles with a thin copper hydroxide coating. In addition, scattering blue vitriol crystals in the clarifier does not produce uniform dosages of copper sulfate in the water. Instead, very high dosages will be found immediately down current from the crystals and little copper sulfate will be found elsewhere.
In this invention, a water soluble copper salt is dissolved in an aqueous solution of an acidic flocculent. It is fortuitous that the selected flocculants are quite acidic because many water soluble copper salts, and the preferred copper sulfate, are much more soluble in low pH water than in neutral to high pH water. The resultant algicide-flocculant solution is commonly delivered in a tank truck or by a tank rail car and off loaded into storage tanks.
The solution is then metered into the clarifier, thus delivering a reliable, predictable quantity of flocculant and copper algicide. This technique operates to deliver a uniform low dose of copper algicide in a very simple, efficient and trouble free manner because flocculent metering pumps deliver a controlled amount of flocculent in a manner which mixes the flocculant into the raw water in a thorough and uniform manner. It is this liquid injection that thoroughly and uniformly mixes the algicide with the water, as contrasted to the prior art technique scattering of blue vitriol crystals with a shovel into the clarifier. Efficient mixing of the copper algicide with the water provides low, uniform dosages of copper which is very desirable because little copper is wasted.
The copper solution provides copper (II) ions that displace the magnesium ion in chlorophyll to kill the algae in the clarifier. The amount of copper in the algicide-flocculant solution is controlled; thus the amount of copper in the clarifier is also controlled and is maintained at low levels. The copper reacts with the magnesium in the chlorophyll molecules and, along with the dead algae, collects in the sludge in the bottom of the clarifier.
When using this invention, no blue copper crystals will be found in the clarifier sludge which means that more of the copper has been put to its intended use of killing algae rather than wasted in the clarifier sludge. In addition, the amount of soluble copper ion passing through the clarifier into the finished water will normally not exceed 0.1 ppm which is well below the 1.3 ppm standard required by the Lead and Copper Rule of the Environmental Protection Agency.
It is difficult to overstate the importance of low, uniform dosages of copper. For the algicide to be effective, copper (II) ions must come intimately close to the magnesium ion in the chlorophyll complex of substantially all of the algae cells. Although there is moderate circulation in the clarifier, it is hard to imagine copper ions in concentrations on the order of less than 1 ppm reacting with enough magnesium atoms in chlorophyll to control an algae bloom. However, this can be done with improved mixing and distribution of the algicide when it is combined with the flocculant as in this invention. Uniform dosages are the key to effectiveness. Low dosages are the key to economy.
In the past, a water plant has typically used a conventional flocculent, either with or without a polymeric flocculant aid. With the onset of a substantial algae bloom, attempts would be made to control the algae bloom using the prior art technique with less than satisfactory results. When facing a full grown algae bloom, the amount of copper in the algicide-flocculant solution of this invention would be at a relatively high level which will bring the algae bloom under control in a fairly short period. After the algae bloom is brought under control, the amount of soluble algicide will be reduced in subsequent batches of algicide-flocculant solutions and ultimately reduced to a lower level that is sufficient to keep algae growth suppressed at a very reasonable cost. A large proportion of the algicidal copper exits the treated water stream in the clarifier sludge and not with the finished water because it has been intimately bonded to the algae chlorophyll.
An object of this invention is to provide an improved technique for treating algae in a municipal water plant.
Another object of this invention is to provide an improved technique for preparing an algicide-flocculant solution.
A more specific object of this invention is to treat raw water with an algicide-flocculant solution which, when added to a water plant clarifier, drops particulates out of suspension and simultaneously controls algae in the clarifier.
These and other objects and advantages of this invention will become more apparent as this description proceeds, reference being made to the appended claims.
The copper algicide of this invention is selected from water soluble copper salts. From a simple algicidal standpoint, almost any water soluble copper salt is suitable. From the standpoint of producing potable water, the choice is more limited because not all water soluble copper salts can economically be put into drinking water. Thus, the common choices for the water soluble copper salt are copper sulfate, copper chloride, copper nitrate and copper acetate. The selection will likely be based on the relative cost of copper salts. Copper sulfate is the preferred water soluble copper salt because it is the only one presently approved for use in municipal water plants; it is effective as a source of copper (II) ions; and, it is the least expensive of the possible candidates. The preferred form of copper sulfate is blue vitriol which is cupric sulfate pentahydrate.
The amount of blue vitriol in the algicide-flocculant solution varies between 0.1-5% by weight. The proportion of copper in blue vitriol is 25.45% by weight which means that the copper concentration in the algicide-flocculant solution varies from about 0.025-1.25%. Preferably, the amount of blue vitriol in the algicide-flocculant solution is 0.1-1% by weight meaning that the active copper concentration in the preferred solution is 0.025-0.25% by weight. The equivalent concentration of other soluble copper salts is found in Table I:
Accordingly, the concentration of the copper salts vary from about 0.05% to about 6% in order to provide the desired range of active copper concentration.
It has been found that an active copper concentration of 0.25% in the algicide-flocculant solution is sufficient to bring algae blooms under control. As initial batches of algicide-flocculant solution are used in response to an algae bloom, the algae die off substantially. Subsequent batches of algicide-flocculant solution preferably contain reduced amounts of active copper algicide, usually about half the initial dose. It has been found that copper concentrations in the algicide-flocculant solution approaching 0.025%, such as in the range of about 0.025-0.050%, are sufficient as a maintenance dose to keep algae under control and prevent the formation of algae blooms, even under the most trying conditions of temperature and sunlight.
Theoretically, it is possible to add copper salt crystals in the proper proportion to an acidic flocculent solution and agitate the solution to dissolve the copper salt. In practice, this has not been efficient for a variety of reasons. The source of blue vitriol is solid pebbles which require vigorous agitation to dissolve, such as occurs with a powered impeller. With aluminum sulfate as the flocculent, solubility is adversely affected by the common ion effect. It is accordingly much better to dissolve the copper salt in water and then mix the water soluble copper salt solution with the flocculant solution. Using copper sulfate as the algicide, one part blue vitriol is dissolved in two parts water making a nearly saturated copper sulfate solution.
To prepare the water solution of the copper salt, a mixing tank is partially filled with water and a suitable mixer, such as a powered impeller, is used to agitate the water. Preferably, the water is heated with a suitable heater, such as an electrically powered immersion heater or preheated with a conventional water heater. The selected copper salt is taken from commercially available bags and the desired quantity added to the tank. Using blue vitriol, the water solution will initially be bluish but somewhat milky which is caused by partial formation of copper hydroxide. Continued stirring and complete dissolving of the blue vitriol will result in a clear blue color typical of copper sulfate solutions.
During the mixing process, a small quantity of acidic flocculent solution is poured into the tank to acidify the copper salt solution to a pH of no more than 5 and preferably in the range of 4-5. A typical flocculant solution has a pH of about 2.5, which is about the same as lemon juice. Acidifying the solution prevents the formation of copper hydroxide so the copper salt completely dissolves, and remains in solution. Acidifying the solution with the flocculant material avoids using a different acid material which, in the treatment of water for human consumption, might provide regulatory problems.
The flocculant of this invention is acidic and, when mixed with the algicide, acidifies the water used to dissolve the copper salt. Suitable flocculants are aluminum sulfate, iron sulfate, iron chloride and mixtures thereof. Preferably, but not necessarily, the flocculants are prepared in a nearly saturated solution. In a typical process, aluminum oxide is reacted with sulfuric acid to produce liquid aluminum sulfate, i.e. about 47-50% by weight aluminum sulfate in water. Iron sulfate or iron chloride may be prepared by commonly known procedures, as is well known in the art. In this invention, the amount of flocculant in the algicide-flocculant solution varies between 25-50% by weight and preferably is 35-50% by weight.
The invention is also useable with polymer flocculant aids of any suitable type. Polymer flocculant aids are long chain, high molecular weight cationic materials, usually having molecular weights in the range of 100,000-800,000. Conventional flocculants, such as aluminum sulfate, iron sulfate, iron chloride and mixtures thereof, produce relatively small flocs which require relatively long residence times to settle out by gravity in the clarifier. The polymer flocculant aids cause these small flocs to agglomerate into larger particles that settle at faster rates, thereby allowing shorter residence times in the clarifier. The present standard polymer flocculant aids are high molecular weight quaternary amines such as diallyldimethylammonium chloride which is commercially available from CPS Chemical Company, West Memphis, Ark. In this invention, the amount of polymer flocculant aid in the algicide-flocculant solution varies between 0-10% by weight but preferably is 0-5% by weight. Preferably, the polymer flocculent aid is added to the flocculant prior to the addition of the water soluble copper salt solution.
Potable water treatment chemicals are typically delivered by tank truck to the water plant. In this invention, the flocculent, with or without the cationic polymer aid, and the acidified copper salt solution are thoroughly mixed in a processing vessel or tank by agitation, air mixing or a recirculating pump. The complete homogenous mixture is then loaded into the tank truck for delivery to the water plant. In an alternate production method, the flocculent, with or without the cationic polymer aid, is simultaneously loaded with the acidified copper salt solution into the tank of a tank truck. Final mixing occurs during transport, caused by agitation o the liquid contents due to movement of the truck.
The algicide-flocculant solution, with or without the polymeric flocculent aid, is added to raw water in the clarifier using conventional metering equipment to deliver sufficient flocculent to coagulate the particulates in the raw water. Typically, nearly saturated flocculant is added to the raw water in the range of 20-60 ppm, an average value being about 30 ppm. Because incoming raw water contains very little soluble copper, the active copper concentration in the clarifier is due almost entirely to the copper algicide combined with the flocculant. Thus, the clarifier water has a copper concentration in the range of 0.050-0.15 ppm.